Are there any violations of the prevailing rhyme scheme of the poem? Can a reason be found for the poet's allowing such aberrations?

The poet would not "allow" any aberrations; he would choose such aberrations for a poetic reason. The rhyme scheme which uses lines 2,4,5,6 of each stanza rhyming with words such as "shore" and "Nevermore" makes the poem interlinked, almost as if the ideas are tied together with, of course, the idea that some things will be "nevermore." Aberrations are usually tied to a change in emotion, so if there any such aberrations, one should look at the emotional state of the narrator and consider whether or not the aberrations have some significance in terms of what the verse is focusing on.